1. Field of the Invention
The present invention relates generally to variable compression ratio internal combustion engines. More particularly, the invention relates to a method and system for determining the torque output of a variable compression ratio internal combustion engine.
2. Background Art
The “compression ratio” of an internal combustion engine is defined as the ratio of the cylinder volume when the piston is at bottom-dead-center (BDC) to the cylinder volume when the piston is at top-dead-center (TDC). Generally, the higher the compression ratio, the higher the thermal efficiency and fuel economy of the internal combustion engine. So-called “variable compression ratio” internal combustion engines have been developed, for example, having higher compression ratios during low load conditions and lower compression ratios during high load conditions. Various techniques have been disclosed for varying compression ratio, including for example, using “sub-chambers and “sub-pistons” to vary the volume of a cylinder, see for example patents U.S. Pat. No. 4,246,873 and U.S. Pat. No. 4,286,552; varying the actual dimensions of all or a portion of a piston attached to a fixed length connecting rod, see U.S. Pat. No. 5,865,092; varying the actual length of the connecting rod itself, see U.S. Pat. Nos. 5,724,863 and 5,146,879; and using eccentric rings or bushings either at the lower “large” end of a connecting rod or the upper “small” end of the connecting rod for varying the length of the connecting rod or height of the reciprocating piston, see U.S. Pat. No. 5,562,068, U.S. Pat. No. 5,960,750, U.S. Pat. No. 5,417,185 and Japanese Publication JP-03092552.
As with conventional internal combustion engines, it is vitally important for a number of reasons to be able to accurately estimate the output torque of a variable compression ratio internal combustion engine. Torque estimates are used, for example, to schedule hydraulic line pressures in a step ratio transmission, prevent transmission braking in certain gears by limiting peak torque, and to coordinate operation of a vehicle's anti-lock braking system so as to minimize wheel slip. In vehicles having multiple torque sources, for example hybrid electric vehicles, torque estimates are required in order to properly coordinate and arbitrate the various torque sources onboard the vehicle.
The inventor herein has recognized the need to accurately determine the output torque as a function of a selected engine compression ratio in order to ensure optimal control and performance of the engine and corresponding motor vehicle.